Boiling water reactor fuel assemblies are commonly used in nuclear reactors throughout the world. During normal reactor operations, the reactor must be stopped to provide new fuel. Generally, a boiling water reactor is operated for a period between 12 and 36 months before the reactor must be shut down. After shutdown, termed an “outage”, approximately ⅓ of the fuel assemblies which comprise the core of the reactor are replaced with new fuel. The new fuel is generally placed in locations in the reactor according to the specific characteristics of the reactor and the fuel assemblies present therein. New fuel is often first placed in the outside areas of the reactor and then moved towards the center of the reactor during each progressive outage occurring thereafter.
Many progressive moves of fuel assemblies are made during the lifetime of the reactor core to allow for efficient operation of the reactor. Over time, the fuel assemblies must also be lifted and removed from the reactor to be inspected for wear and mechanical damage. As is evident, handling of fuel assemblies is critical to safe and continued operation of a nuclear facility.
In current boiling water reactor fuel assemblies each fuel rod rests on a lower tie plate. The weight of all the fuel rods, lower tie plate and other structural components is transferred to internal tie rods when a fuel assembly is lifted. The internal tie rods prevent overstress of the relatively fragile fuel rods. Other configurations for boiling water reactor fuel assemblies use an internal water channel to carry the weight of the assembly.
Other configurations of fuel assemblies use the outside fuel channel to aid in lifting the assembly. These configurations are known as “bundle in basket” designs. In these assemblies, inspection of the interior parts, such as the fuel rods, is performed by holding the outside fuel channel in a stationary position, disconnecting the fuel channel from the inner fuel rods, and lifting the interior fuel rods out of the fuel channel for inspection. These designs require that the heavy part of the fuel assembly, namely the fuel rods and spacer grids, be lifted to produce a separation between the fuel assembly outside fuel channel and the fuel assembly internals. There are many drawbacks to this type of configuration. The fuel rods are lifted to facilitate the inspection, thereby increasing chances of a lifting accident involving the fissile material. Since the fissile material must be lifted with this configuration, the overhead crane systems or other mechanisms used to perform the lift must undergo stringent safety checks and have specific configurations to enable the lift to occur. The lift of the heavy internal structure sometimes results in damage to the relatively fragile spacer grid configuration along the sides of the fuel assembly. This damage occurs when the fuel assembly is “re-channeled” or reinserted into the outside fuel channel. The lowering of the fuel assembly into the relatively narrow fuel channel often produces binding or impact between the spacer grids and fuel channel. If damage occurs, the damaged spacer grids must be inspected and/or evaluated to determine the extent of the damage and applicability of future use. The lowering of the fissile material into the fuel channel is accomplished at slow speeds, in an attempt to avoid damage to the fuel spacer grid. This slow speed increases fuel reloading time and increases economic cost for the nuclear facility.
There is a need to limit handling of heavy loads containing fissile material to increase safety in a nuclear facility.
The is also a need to limit expensive lifting and/or rigging mechanisms and their use in conjunction with nuclear fuel assemblies.
There is a further need to limit damage to fuel assemblies when an assembly is re-channeled.
There is a still further need to speed refueling of a nuclear reactor to increase economic profitability.